Exercises set 1

Classes of sets (Lectures 1, 2, 3 and 4)

1.1. Semi-algebra and algebra of sets (1.1) Let F be any collection of subsets of a set X. Show that F is an algebra if and only if the following hold: (i) ∅, X ∈ F. (ii) Ac ∈ F whenever A ∈ F. (iii) A ∪ B ∈ F whenever A, B ∈ F. (1.2) Let F be an algebra of subsets of X. Show that (i) If A, B ∈ F then A△B := (A \ B) ∪ (B \ A) ∈ F. (ii) If E1, E2,...,En ∈ F then there exists sets F1, F2,...,Fn ∈ F such ∞ ∞ that Fi ⊆ Ei for each i, Fi ∩ Fj = ∅ for i 6= j and i=1 Ei = j=1 Fj. The next set of exercise describes some methods of constructS ingS algebras and semi-algebras. (1.3) Let X be a nonempty set. Let ∅= 6 E ⊆ X and let C be a semi-algebra (algebra) of subsets of X.

C∩ E := {A ∩ E | A ∈ C}. Note that C∩ E is the collection of those subsets of E which are elements of C. Show that C∩ E is a semi-algebra (algebra) of subsets of E. (1.4) Let X,Y be two nonempty sets and f : X −→ Y be any map. For E ⊆ Y, we write f −1(E) := {x ∈ X | f(x) ∈ E}. Let C be any semi- algebra (algebra) of subsets of Y. Show that f −1(C) := {f −1(E) | E ∈ C}

1 2 1. Classes of sets (Lectures 1, 2, 3 and 4)

is a semi-algebra (algebra) of subsets of X. (1.5) Give examples of two nonempty sets X,Y and algebras F, G of subsets of X and Y, respectively such that F × G := {A × B | A ∈ F,B ∈ G} is not an algebra. (It will of course be a semi-algebra.

(1.6) Let {Fα}α∈I be a family of algebras of subsets of a set X. Let F := α∈I Fα. Show that F is an algebra of subsets of X. Is F a semi-algebra of subsets of X If each Fα a semi-algebra? T (1.7) Let {Fn}n≥1 be a sequence of algebras of subsets of a set X and F := ∞ n=1 Fn. In general, F is not an algebra. Under what conditions on Fn can you conclude that is also an algebra? S (1.8) Let C be any collection of subsets of a set X. Then there exists a unique algebra F of subsets of X such that C ⊆ F and if A is any other algebra such that C⊆A, then F⊆A. This unique algebra is called the algebra generated by C and is denoted by F(C). (1.9) Show that he algebra generated by I, the class of all intervals, is n {E ⊆ R | E = k=1 Ik,Ik ∈ I, Ik ∩ Iℓ = ∅ if 1 ≤ k 6= ℓ ≤ n}. (1.10) Let C be anyS semi-algebra of subsets of a set X. Show that F(C), the algebra generated by C, is given by n {E ⊆ X | E = Ci,Ci ∈C and Ci ∩ Cj = ∅ for i 6= j, n ∈ N}. i [=1 This gives a description of F(C), the algebra generated by a semi-algebra C. In general, no description is possible for F(C) when C is not a semi- algebra. (1.11) Let X be any nonempty set and C = {{x} | x ∈ X} {∅, X}. Is C a semi-algebra of subsets of X? What is the algebra generated by C? Does S your answer depend upon whether X is finite or not? (1.12) Let C be any collection of subsets of a set X and let E ⊆ X. Let C∩ E := {C ∩ E | C ∈ C}. Then the following hold: (a) C∩ E ⊆ F(C) ∩ E := {A ∩ E | A ∈ F(C)}. Deduce that F(C∩ E) ⊆ F(C) ∩ E. (b) Let A = {A ⊆ X | A ∩ E ∈ F(C∩ E)}. Then, A is an algebra of subsets of X, C⊆A and A∩ E = F(C∩ E). 1.2. Sigma algebra and monotone class 3

(c) Using (a) and (b), deduce that F(C) ∩ E = F(C∩ E).

Optional Exercises

(1.13) Let C be a semi-algebra of subsets of a set X. A set A ⊆ X is called a σ-set if there exist sets Ci ∈ C, i = 1, 2,..., such that Ci ∩ Cj = ∅ for ∞ i 6= j and i=1 Ci = A. Prove the following: n (i) For any finite number of sets C,C1,C2,...,Cn in C, C \ ( Ci) is S i=1 a finite union of pairwise disjoint sets from C and hence is a σ-set. ∞ S (ii) For any sequence {Cn}n≥1 of sets in C, n=1 Cn is a σ-set. (iii) A finite intersection and a countable union of σ-sets is a σ-set. S (1.14) Let Y be any nonempty set and let X be the set of all sequences with elements from Y, i.e.,

X = {x = {xn}n≥1 | xn ∈ Y,n = 1, 2,...}. For any positive integer k let A ⊆ Y k, the k-fold Cartesian product of Y with itself, and let i1 < i2 < · · · < ik be positive integers. Let

C(i1, i2,...,ik; A) := {x = (xn)n≥1 ∈ X | (xi1 ,...,xik ) ∈ A}.

We call C(i1, i2,...,ik; A) a k-dimensional cylinder set in X with base A. Prove the following assertions: (a) Every k-dimensional cylinder can be regarded as a n-dimensional cylinder also for n ≥ k. (b) Let A = {E ⊂ X | E is an n-dimensional cylinder set for some n}. Then, A ∪ {∅, X} is an algebra of subsets of X.

1.2. Sigma algebra and monotone class (1.15) Let S be a σ-algebra of subsets of X and let Y ⊆ X. Show that S∩ Y := {E ∩ Y | E ∈ S} is a σ-algebra of subsets of Y. (1.16) Let f : X → Y be a function and C a nonempty family of subsets of Y . Let f −1(C) := {f −1(C) | C ∈ C}. Show that S(f −1(C)) = f −1(S(C)). (1.17) Let X be an uncountable set and C = {{x} | x ∈ X}. Identify the σ-algebra generated by C. (1.18) Let C be any class of subsets of a set X and let Y ⊆ X. Let A(C) be the algebra generated by C. (i) Show that S(C)= S(A(C)). (ii) Let C∩ Y := {E ∩ Y | E ∈ C}. Show that S(C∩ Y ) ⊆ S(C) ∩ Y. (iii) Let S := {E ∪ (B ∩ Y c) | E ∈ S(C∩ Y ),B ∈ S(C)}. 4 1. Classes of sets (Lectures 1, 2, 3 and 4)

Show that S is a σ-algebra of subsets of X such that C ⊆ S and S∩ Y = S(C∩ Y ). (iv) Using (i), (ii) and (iii), conclude that S(C∩ Y )= S(C) ∩ Y. (1.19) Let X be any topological space. Let U denote the class of all open subsets of X and C denote the class of the all closed subsets of X. (i) Show that S(U)= S(C). This is called the σ-algebra of Borel subsets of X and is denoted by BX . (ii) Let X = R. Let I be the class of all intervals and I˜ the class of all left-open right-closed intervals. Show that I ⊂ S(U), I ⊂ S(I˜), I˜ ⊂ S(I) and hence deduce that

S(I)= S(I˜)= BR. (1.20) Prove the following statements: (i) Let Ir denote the class of all open intervals of R with rational endpoints. Show that S(Ir)= BR. (ii) Let Id denote the class of all subintervals of [0, 1] with dyadic endpoints (i.e., points of the form m/2n for some integers m and n). Show that S(Id)= BR ∩ [0, 1]. (1.21) Let C be any class of subsets of X. Prove the following: (i) If C is an algebra which is also a monotone class, show that C is a σ-algebra. (ii) C ⊆ M(C) ⊆ S(C). (1.22) (σ-algebra monotone class theorem ) Let A be an algebra of subsets of a set X. Then , S(A)= M(A). Prove the above statement by proving the following: (i) M(A) ⊆ S(A). (ii) Show that M(A) is closed under complements by proving that for B := {E ⊆ X | Ec ∈ M(A)}, A⊆B, and B is a monotone class. Hence deuce that M(A) ⊆B. (iii) For F ∈ M(A), let L(F ) := {A ⊆ X | A ∪ F ∈ M(A)}. Show that E ∈ L(F ) iff F ∈ L(E), L(F ) is a monotone class, and A⊆L(F ) whenever F ∈A. Hence, M(A) ⊆L(F ), for F ∈A. (iv) Using (iii), deduce that M(A) ⊆ L(E) for every E ∈ M(A),i.e., M(A) is closed under unions also. Now use exercise (1.22) to deduce that S(A) ⊆ M(A).